1. a. The primary long-term goal of this study is described the way in which neurogenic, biogenic and local mechanical-physical features of the microvessel wall interact in regulating microcirculation, in conventional young and aged rats, germfree and hypertensive rats. b. A quantitative assessment will be made of the influence of selected naturally occurring biogenic molecules on microvascular response to central and peripheral neural stimulation in mesentery, skeletal muscle and pial microvasculatures; 2. Venular distensibility gradients resulting in elicitaion in arteriolar and precapillary sphincter reponse will be determined. The consequence of the microvascular diametric response on flow will be quantitatively assessed. 3. The extent, if any, of other central neural loci of transmitter(s) release e.g., nucleous coereleus, on cerebral (pial, cortical), and other tissue microcirculation will be examined.